**Background:** Alpha-1 antitrypsin deficiency (AATD) is one of the most common inherited rare diseases in Caucasians, with a prevalence of 1–5/10,000. The clinical presentation of AATD is highly variable, ranging from early-onset emphysema and bronchiectasis to liver cirrhosis, panniculitis, and vasculitis. While the primary cause is pathogenic variants in the SERPINA1 gene, the natural history and disease progression are unpredictable and only partially explained by known risk factors such as smoking or occupational exposures. A protease–antiprotease imbalance is hypothesized as the primary mechanism of lung destruction, but this does not fully account for the wide phenotypic variability. This narrative review summarizes current evidence on genetic modifiers and prognostic markers that may contribute to the different respiratory phenotypes of AATD.
**Methods:** The review synthesizes findings from multiple studies investigating genetic variants, epigenetic modifications, and post-transcriptional regulators in AATD cohorts. Studies include candidate gene analyses, family-based association studies, genome-wide association studies (GWAS), whole-exome sequencing (WES), DNA methylation arrays, and miRNA expression profiling. Key cohorts include the AAT Genetic Modifiers Study (378 PI*ZZ individuals from 167 families), the UK AATD national registry (458 subjects), the Boston Early-Onset COPD Study (369 subjects), the International COPD Genetics Network (1085 patients), and several smaller case-control and sibling studies.
**Key Results:** The review categorizes genetic modifiers into four groups: (1) Single nucleotide polymorphisms (SNPs): NOS3 774T and 894T alleles were more frequent in severely affected AATD individuals (0.417 vs. 0.289 and 0.427 vs. 0.344, respectively). GSTP1 105Val frequency was significantly increased in AATD vs. healthy controls (OR 2.09, 95% CI 1.17–3.72) and vs. COPD patients (OR 2.41, 95% CI 1.27–4.59). TNF-α rs361525 was associated with chronic bronchitis in PI*ZZ subjects (genotype p=0.01, allele p=0.01). IL10 SNPs (rs1800871, rs1518110) showed consistent associations with airflow obstruction. IREB2 rs2568494, LOC123688 rs8034191, and CHRNA3 rs1051730 were associated with pre-bronchodilator FEV1%pred (p<0.05). WES in four families with extreme phenotypes identified 15 variants in 14 genes (57% immune-related) under a recessive model and 23 variants in 21 genes (29% immune-related) under a dominant model in affected vs. non-affected AATD siblings. (2) DNA methylation: Two CpG sites in SERPINA1 (cg02181506 and cg24621042) were identified as the highest-ranking methylation marks associated with COPD in two independent cohorts. However, subsequent studies showed conflicting results: Sundar et al. found no significant SERPINA1 hypomethylation in smokers/COPD, and Beckmeyer-Borowko et al. found no association after multiple testing corrections in 1076 adults and 259 children. Rotondo et al. reported SERPINA1 hypermethylation in 80% of COPD+ vs. 56.5% of COPD- acute coronary syndrome patients (p<0.05). (3) microRNAs: miR-199a-5p was the most upregulated miRNA in asymptomatic PI*ZZ vs. PI*MM individuals and modulates UPR components. miR-320c expression was elevated in 98 individuals with pulmonary disease regardless of AAT level. hsa-miR-335-5p was downregulated in severe vs. mild PI*ZZ COPD patients, with 12 target genes in cytokine, MAPK/mk2, and JNK signaling pathways upregulated. (4) SERPINA1 mRNA isoforms: The gene produces 11 mRNA isoforms via alternative splicing in the 5'-UTR, with a proposed posttranscriptional regulatory program involving upstream open reading frames (uORFs) and RNA structure.
**Clinical Implications:** Despite extensive research, no definitive genetic modifier or prognostic biomarker for AATD pulmonary phenotypes has been validated for clinical use. The review identifies several limitations across studies: small cohort sizes, poor matching, lack of independent replication, and contradictory findings (e.g., GSTP1 and NOS3 associations confirmed in some studies but not others). The authors emphasize that clinical significance, not just statistical significance (p≤0.05), should guide interpretation, particularly in rare disease research. They recommend prospective data collection from international registries such as the European EARCO registry. The potential role of TNF in AATD is highlighted as worthy of further investigation given AAT's immunoregulatory functions independent of antineutrophil elastase activity. The review also notes a shortage of clinical data on rare SERPINA1 variants beyond PI*Z and PI*S, and calls for efforts to collect data on the clinical significance of largely unknown variants.